Immunity can generally be classified as innate immunity or as adaptive immunity. Innate immune responses typically occur immediately upon infection to provide an early barrier to infectious disease, whereas adaptive immune responses occur later with the generation of antigen-specific effector cells and immunological memory. Innate immune responses do not generate lasting protective immunity, but appear to play a role in the generation of later arising adaptive immune responses.
Toll-like receptors (TLRs) are essential for innate immune responses as they recognize several different antigens and initiate immune responses (e.g., activation of dendritic cells and macrophages, and cytokine production). TLRs are type-I transmembrane proteins that recognize a variety of pathogen-associated molecular patterns (PAMPs) from bacteria, viruses and fungi. In this way PAMPs serve as a first-line of defense against invading pathogens. Human TLRs can elicit overlapping yet distinct biological responses due to differences in cellular expression and activation of downstream signal transduction pathways (Akira et al., Adv. Immunol. 78: 1-56, 2001).
TLRs are characterized by an ectodomain composed of leucine-rich repeats and a cytoplasmic domain, known as a Toll/interleukin-1 receptor domain. The ectodomain is responsible for recognition of PAMPs, while the cytoplasmic domain is required for downstream signaling. TLRs usually undergo dimer formation and/or a conformation change to activate downstream signal transduction pathways. Studies have shown that LRR8 is involved in DNA and RNA recognition, whereas LRR17 is involved in nucleic acid binding (Smits et al., Oncologist, 13: 859-875, 2008).
The family of TLRs consists of ten members in human (TLR1-TLR10) and twelve members in mice (TLR1-TLR9 and TLR11-TLR13). The TLRs that are located in the plasma membrane recognize bacterial membrane components, whereas the TLRs that detect nucleic acid-based ligands are predominately located within endosomal compartments. The nucleic acid-sensing TLRs include TLR3, TLR7, TLR8, and TLR9. Upon ligand-binding, TLRs initiate a signal transduction cascade leading to activation of NFκB through the adapter protein myeloid differentiation primary response gene 88 (MyD88) and recruitment of the IL-1 receptor associated kinase (IRAK). Phosphorylation of IRAK in turn leads to the recruitment of TNF-receptor associated factor 6 (TRAF6), which results in the phosphorylation and degradation of the NF-κB inhibitor, I-κB, thereby releasing NF-κB. NF-κB enters the cell nucleus and initiates gene transcription, leading to production of proflammatory cytokines, chemokines, and type I interferons (IFNs), as well as the upregulation of costimulatory molecules.
TLR8 belongs to the same subfamily as the TLR7 and TLR9 endosomal receptors and is highly homologous to TLR7 (Liu et al., Mol. Immunol. 47:1083-90, 2010). The role of TLR8, and of its close homologue TLR7, is to detect the presence of “foreign” single-stranded RNA within a cell, as a means to respond to viral invasion (Heil et al., Science 303:1526, 2004; and Diebold et al., Science 303:1529, 2004). While the TLR8 gene in humans is closely related to TLR7, TLR8 has distinct, but overlapping specificity for RNA and synthetic small molecules with a structure related to nucleic acids (Medzhitov et al., Immunol. Rev. 173:89-97, 2000). Some ssRNA synthetic sequences containing repetitive A/U motifs are able to specifically activate TLR8 but not TLR7 (Gorden et al. J. Immunol. 174:1259-68, 2005). Further, in humans, TLR8 is highly expressed in monocytes, macrophages, myeloid dendritic cells (mDC) and neutrophils, whereas TLR7 in blood cells is principally expressed in pDCs, B-cells, and neutrophils. Because of this difference in cellular expression, triggering by RNA through TLR7 in blood leads to a response dominated by Type I IFN production, whereas activation through TLR8 induces multiple pro-inflammatory cytokines: TNF, IL-12, IL-6, IL-8 and IL-1 (Barrat et al., J Exp Med. 2202:1131-9, 2005; and Gorden et al., J. Immunol. 174:1259-68, 2005).
Although TLR8 polymorphisms have been associated with some autoimmune diseases, the role of TLR8 and its specific ligands has not been clearly defined. One key limitation in elucidating TLR8 biology is the lack of an animal model. For example, mouse TLR8 has a very different specificity than human TLR8. Mouse TLR8 lacks the ability to respond to ssRNA ligands, RNA viruses or small molecules; all of which have been shown to activate human TLR8 (Heil et al. Science 303:1526-9, 2004; Jurk et al. Nat Immunol 3:499, 2002; Hemmi et al. Nat. Immunol 3:196-200, 2002; and Lund et al. PNAS 101:5598-603, 2004). Further, by comparing the amino acid sequence, TLR8 of mice and rats lack a five amino-acid sequence required for ligand recognition in man (Liu et al. Mol. Immunol. 47:1083-90, 2010). The lack of useful animal models and the very different ligand specificities of human TLR8 and its rodent orthologs have proven to be major limitations to the study of TLR8 biology.